Van der Waals forces, relatively weak electricforces that attract neutral molecules to one another in gases, in liquefied and solidified gases, and in almost all organic liquids and solids. The forces are named for the Dutch physicist Johannes Diderik van der Waals, who in 1873 first postulated these intermolecular forces in developing a theory to account for the properties of real gases. Solids that are held together by van der Waals forces characteristically have lower melting points and are softer than those held together by the stronger ionic, covalent, and metallic bonds.

Van der Waals forces may arise from three sources. First, the molecules of some materials, although electrically neutral, may be permanent electric dipoles. Because of fixed distortion in the distribution of electric charge in the very structure of some molecules, one side of a molecule is always somewhat positive and the opposite side somewhat negative. The tendency of such permanent dipoles to align with each other results in a net attractive force. Second, the presence of molecules that are permanent dipoles temporarily distorts the electron charge in other nearby polar or nonpolar molecules, thereby inducing further polarization. An additional attractive force results from the interaction of a permanent dipole with a neighbouring induced dipole. Third, even though no molecules of a material are permanent dipoles (e.g., in the noble gasargon or the organic liquid benzene), a force of attraction exists between the molecules, accounting for condensing to the liquid state at sufficiently low temperatures.

The weak dipole attraction of the van der Waals bond.

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The nature of this attractive force in molecules, which requires quantum mechanics for its correct description, was first recognized (1930) by the Polish-born physicist Fritz London, who traced it to electron motion within molecules. London pointed out that at any instant the centre of negative charge of the electrons and the centre of positive charge of the atomic nuclei would not be likely to coincide. Thus, the fluctuation of electrons makes molecules time-varying dipoles, even though the average of this instantaneous polarization over a brief time interval may be zero. Such time-varying dipoles, or instantaneous dipoles, cannot orient themselves into alignment to account for the actual force of attraction, but they do induce properly aligned polarization in adjacent molecules, resulting in attractive forces. These specific interactions, or forces, arising from electron fluctuations in molecules (known as London forces, or dispersion forces) are present even between permanently polar molecules and produce, generally, the largest of the three contributions to intermolecular forces.

...molecular compounds occur as solids. The role of weak intermolecular forces in the properties of gases was first examined theoretically by the Dutch scientist Johannes van der Waals, and the term van der Waals forces is used synonymously with intermolecular forces. Under certain conditions, weakly bonded clusters of molecules (such as an argon atom in association with a hydrogen chloride...

Neutral molecules may be held together by a weak electric force known as the van der Waals bond. It results from the distortion of a molecule so that a small positive charge develops on one end and a corresponding negative charge develops on the other (see Figure 7D). A similar effect is induced in neighbouring molecules, and this dipole effect propagates throughout...